In recent years, the digital versatile disc (DVD) has been attracting attention as a large capacity optical recording medium since it can record digital information at a recording density about six times higher than that of the compact disc (CD). However, in accordance with the increase in capacity of information, higher density optical recording media have been demanded. In order to achieve higher density than DVD (wavelength: 660 nanometers (nm) and numerical aperture (NA): 0.6), it is necessary to shorten the wavelength of a light source and to increase the NA of the objective lens. For example, when a blue laser with the wavelength of 405 nm and an objective lens having the NA of 0.85 are used, a recording density five times higher than that of DVD can be achieved. Furthermore, by making a recording layer of an optical recording medium to have two-layered structure, the recording capacity becomes an additional two times higher than that of DVD.
However, in the high-density optical recording medium using the above-mentioned blue laser, in order to increase the recording capacity, the track pitch becomes quite narrow. Therefore, in order to make the tracking error signal stable, it is necessary to increase the rim intensity, i.e., a ratio of the central intensity to the peripheral intensity of light with which an optical recording medium is irradiated. The rim intensity can be increased by using only the central portion of light emitted from a light source. However, the use efficiency (capture efficiency) of light emitted from the light source decreases. Therefore, in order to record information in the optical recording medium, it is necessary to use a light source capable of outputting a large amount of light. Furthermore, in order to record information in a multi-layered optical recording medium, a light source capable of outputting an even larger amount of light is necessary. However, a light source capable of outputting an even large amount of light has a problem in lifetime and yield is bad. Therefore, an optical element having the large rim intensity and still having high capture efficiency is proposed in JP11 (1999)-258544A.
FIG. 10 is a view to explain a configuration of a conventional optical element 90. The optical element 90 includes a first curved surface 83 formed at the side on which light is incident, a second curved surface 84 formed at the side from which light is emitted. The first curved surface 83 and the second curved surface 84 are formed along the surface substantially perpendicular to a central axis line 82, and a peripheral surface 85 is formed so that it connects between the first curved surface 83 and the second curved surface 84 along the direction parallel to the central axis 82. Furthermore, FIG. 10 shows a large number of optical paths of rays of light, passing through the conventional optical element 90 while being refracted, as a plurality of bent lines. The optical element 90 is made of a transparent material (for example, glass) having an isotropic refractive index.
The operation of the thus configured optical element 90 will be explained. The light incident on the first curved surface 83 diverges due to the refraction at the first curved surface 83 in one region inside the optical element 90, and converges in another region of the optical element 90. Therefore, light is emitted through the curved surface 84 with a light intensity distribution that is different from the light intensity distribution of the incident light.
Specifically, in a region Z, light paths of rays of light passing through the optical element 90 extend in parallel to each other. In the central region X at an inner side of the region Z, rays of light diverge. In the peripheral region Y at an outer side of the region Z, rays of light are converged.
Therefore, as shown in the Gaussian luminous intensity distribution W91 shown in the left side of FIG. 10, rays of light with a high intensity located in the central part diverge in passing through the optical element 90, and the intensity thereof is lowered when they are emitted from the optical element 90. In the Gaussian luminous intensity distribution W91, rays of light with a low intensity located at peripheral parts converge in passing through the optical element 90, and the intensity thereof is increased when they are emitted from the optical element 90. Thus, incident light having the Gaussian luminous intensity distribution W91 is converted into emitted light having a uniform luminous intensity distribution W92 as a whole by passing through the optical element 90.
If the thus configured optical element 90 is mounted on an optical head, since it is possible to improve the capture efficiency and to increase the rim intensity, a stable tacking error signal can be achieved with respect to a high density optical recording medium. Furthermore, it is possible to use a light source emitting a small amount of light.
However, when the thus configured light element 90 is mounted on the optical head, strict mounting accuracy is required. Therefore, it is difficult to fabricate an optical head and furthermore, the reliability of the optical head becomes a significant problem. This will be mentioned in detail.
When a divergence angle of a light source is 25°, a focal distance of a collimator lens for taking light emitted from the light source and collimating the emitted light into substantially parallel light is 6.7 millimeters (mm), the diameter of light incident in the optical element is 2.84 millimeters (mm), and the central thickness of the optical element 90 is 1.5 millimeters (mm), the shapes of the first curved surface 83 and the second curved surface 84 are designed so that the rim intensity is increased from 52% to 100%. Then, when the optical element that satisfies these conditions is mounted on the optical head, the optical head 90 may tilt at about 0.1° as a mounting accuracy.
When calculating the case where the optical element 90 designed based on the above-mentioned conditions tilts at 0.1°, since as much as 350 mλ of third-order coma aberration occurs, it is not possible to fabricate the optical head. Furthermore, even if the optical head is fabricated by completely adjusting so as not to tilt, such a large aberration as about 0.1° occurs, and it is not possible to secure the reliability of an optical head.
Furthermore, there is also a problem about decentering between the first curved surface 83 and the second curved surface 84 of the optical element 90 itself. When this optical element 90 is molded, 5 micrometers (μm) of decentering may occur with respect to the accuracy of die for molding. Therefore, when the decentering between the first curved surface 83 and the second curved surface 84 of the optical element 90 is 5 micrometers (μm), 100 mλ of coma aberration occurs. As a result, it is not possible to mount the optical element 90 on the optical head, and thus the yield of the optical element 90 is deteriorated.
With the foregoing in mind, it is an object of the present invention to provide an optical element having the high capture efficiency and high rim intensity, an optical head, a method for correcting a spherical aberration and an optical recording/reproducing apparatus.